ci: move the whole workspace directory on gha/linux
ci: move the whole workspace directory on gha/linux

Moving just the `obj` directory created problems with mountpoints and
Docker containers, so this tries to symlink the parent directory.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust